Electroluminescent display panel

ABSTRACT

An electroluminescent display device comprised of an electroluminescent layer having a large area front electrode and a plurality of back electrodes in which coatings operatively associated with the back electrodes eliminate dark gaps between the light emitting electrodes.

United States Patent Szepesi et al.

[451 Mar. 21, 1972 ELECTROLUMINESCENT DISPLAY PANEL Inventors: Zoltan P. J. Szepesi; Michael A. Novice,

both of Elmira, NY.

Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Sept. 15, 1969 Appl. N0.: 857,751

US. Cl ..117/212, 117/33.5 E, 117/217,

313/108 Int. Cl ..HOlj 1/70, B44d 1/18 Field ofSearch ..313/108; l17/33.5,33.5 A,

Primary Examiner-Alfred L. Leavitt Assistant Examiner-Alan Grimaldi AttorneyF. H. Henson and C. F. Renz [5 7] ABSTRACT An electroluminescent display device comprised of an electroluminescent layer having a large. area front electrode and a plurality of back electrodes in which coatings operatively associated with the back electrodes eliminate dark gaps between the light emitting electrodes.

7 ,9 C'aimw D ntfieqrei 1 1. V

arch 21, 1972 Patented FIG. 2

PRIOR ART FIG. 4

INVENTORS Zol'ron PJ. Szepesi 8 Michael A. Novice am WITNESSES I m? ATTORNE Y ELECTROLUMINESCENT DISPLAY PANEL BACKGROUND OF THE INVENTION minescent layer. Excitation is applied to the front electrode and to selected back electrodes. The back electrodes are spaced apart either for display reasons or insulating reasons and the result is that only the area of the electroluminescent layer immediately beneath the back electrode and between the back electrode and the front electrodeis illuminated upon excitation. When two adjacent electrodes are excited, a dark band or region exists between the illuminated back electrodes areas. In some display configurations, such as bar graphs, nu-

meric or alpha-numeric panels the elimination of this dark gap would improve the presentation and resolution of display panels and several methods have'been proposed in the past to achieve removal.

SUMMARY OF THE INVENTION This invention is directed to the incorporation of a resistance material operatively associated with the back electrodes on an electroluminescent panel to enlarge the excited light area so that it exceeds that of the area of the back electrode. This is accomplished by providing a resistive coating of a material in the area between the electrodes. Best results are obtained with materials whose resistance is responsive to voltage or to light in such a way that itsresistance decreases when two adjacent electrodes are activated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a display panel specifically that of a part of a bargraph incorporating the teachings of this invention;

FIG. 2 is a sectional viewof a portion of the device shown in FIG. 1 taken along the line II-II;

FIG. 3 is an illustration of the prior art; and

FIG. 4 is an illustration of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring in detail to FIGS. 1 and 2, a simple bargraph type display panel is illustrated in which only four back electrodes 21, 22, 23 and 24 are illustrated in order to simplify the explanation of the invention. Itshould be understood that the invention is applicable to many various forms of electroluminescent display devices and different types of back electrodes and front electrodes. The panelelectrode 10 illustrated in FIGS. 1 and 2 is comprised of a light transmissive support plate 12 which may be of any suitable material such as glass and which is transmissive to light emitted from an electroluminescent layer 18. A substantially light transmissive coating 14 is provided on the one surface of the support plate 12 and may be of a suitable material such as stannic oxide. The layer 14 may have a thickness of about 800 nm. and a resistance of about 100 ohms per square. The layer 14 may be referred to as the front electrode and is connected to a suitable potential source 16 by a lead 15. Thepotential source 16 is only an alternating current supply having a frequency of 60 to 2,000 Hertz and a voltage of about 50to 300 volts. The electroluminescentphosphor coating 18- is provided on the conductive coating 14 and may be of any suitable electroluminescent phosphor, such as, zincsulfide combined with suitable activators, such as copper or manganese, as well as suitable fluxes" such as chlorine or bromines anddispersed in a suitable organic material of high dielectric constant.

Positioned on the opposite surface of the electroluminescent phosphor layer 18 are the conductive back electrodes 21, 22, 23 and 24. These coatings 21, 22, 23 and 24 may be provided on the electroluminescent layer 18 by evaporation of a suitable electrically conductive material such as aluminum or gold through a suitable mask structure to provide a coating of about 1 gm thickness and a resistance of about 10 ohms per square. Each of the back electrode contacts 21 through 24 are connected respectively through suitable electrical conductive means and switches 31, 32, 33, 34 respectively to the potential source 16. Again the device has been simplified for purposes of explanation to indicate that the contacts 21 through 24 may be selectively excited in any desired manner.

A resistive coating 26 is then provided over the surface of the electroluminescent layer 18 in such a way that it either bridges the adjacent contacts (21 through 24) or completely covers them. The resistive coating 26 may be of any suitable material such as SiO/Cr whose resistance can be synthesized and adjusted to the required value. It is obvious that the resistance provided by the coating 26 between the contacts should be of a low enough value when two adjacent contacts are excited so as to not cause too high a voltage drop in the area between the contacts. On the other hand, the resistance of the coating 26 between an excited contact and an unexcited contact should be great enough or at least several times higher than the impedance of the connected elements. The resistance between an unexcited and an excited contact should be greater than the impedance of the electroluminescent element i.e., l/wC, where C is the capacitance between a back contact and the front electrode 14 and w is the circular frequency.

Thus the resistance of the coating 26 will depend on the driving frequency of the display panel, on the size of the individual electrodes, and also on the thickness of the electroluminescent layer. Considering for example a practical bargraph panel, the bars of which are 1 inch long, 0.025 inch wide and the separation of the bars is 0.006 inch the following data applies when the driving voltage is 200 v. and the frequency is 400 Hz.: The impedance of one element of the electroluminescent layer under one contact works out to be about 10 ohms, assuming a capacitance value C=250 pf./cm. The resistance between two gaps could be 5 times higher, i.e., 5X10 ohms. The impedance of the electroluminescent layer of the half gap area which is about 1/10 of one electrode area would be about 10 ohms. The half of the resistive coating 26 which is in series with this impedance has about 4 times less resistance (l0 /2.5 10), thus satisfying the requirements for the resistance layer as discussed above. In practice, as the current is distributed across this gap, the voltage even at the middle of the gap will be nearly the same as the common voltage of the two electrodes.

In the above example the resistance of the resistive coating 26 between two electrodes (with 0.006-inch gap) was 5X10 ohms. This means that the resistive coating has a resistance of 8.3 l0 ohms/square. For frequencies other than 400 Hz. the required resistivity will be inversely proportional to the frequency. Also it has to be modified for different sizes of the electrodes and gap areas.

Since the voltage drop in the gap area should be small when both adjacent electrodes are connected and could be high, when only one electrode is connected, the layer 26 should ideally have the property of assuming a low resistance in a low electric field and a high resistance of a high field.

From the above analysis it is apparent that the requirements for a most appropriate resistive layer would be less demanding if it would have a sublinear current versus voltage characteristic, that is the current flowing between the two contacts would be equal to KV" where n is less than 1. This of course means that with an increase in voltage the resistance increases. There are several nonlinear resistance materials or semiconductive materials which exhibit this property, for example, certain varieties of manganese or iron oxide.

The above description has described the utilization of a resistive material which is responsive to a voltage. It is also possible to utilize a resistive material for the coating 26 that is responsive to radiations generated within the electroluminescent layer. Such a material is referred to as a photoconductive material in which the resistance of the material decreases in response to excitation. A suitable photoconductive material is cadmium sulfide. The photoconductive layer may be a plastic embedded powder ofa thickness of 1 mil.

In FIG. 3, the excitation area of a prior art type device is indicated by the lined portion 35 within the electroluminescent layer 18. By utilizing a resistance layer 26 as shown in FIG. 4, the illuminated region is extended to a larger area including the portion 35 and the surrounding portion 37 greater than the area of the back electrode 21. By proper choice of the thickness and the material utilized in this application the region can be made so that it extends to about half of the distance between the two electrodes 23 and 24 as shown in FIG. 4 and thereby will substantially remove the dark gap between two excited contacts.

In the above explanation it is assumed that the back electrodes that are not activated would be normally floating. It is possible that these unexcited contacts could be connected to the same potential as the front electrode 14. In this case, the only limitation would then be the dissipation limit on the resistance.

Various modifications may be made within the spirit of this invention.

We claim as our invention:

1. An electroluminescent display device comprising a layer of electroluminescent phosphor exhibiting the property of emission of light in response to excitation, said layer of electroluminescent material having a front electrode on a first side of said layer and a plurality of back electrode members on a second side of said electroluminescent layer, a resistive coating of material on said second side of said electroluminescent layer covering at least a portion of the region of said second side of said electroluminescent layer and joining at least two of said back electrodes, said resistance coating exhibiting the property of a decrease in resistance in response to external excitation When excitation is applied to said two back electrodes and said front electrode to enlarge the region of illumination within said electroluminescent layer between the said two back electrodes, said resistance coating between adjacent back electrodes having an impedance in the unexcited state greater than the impedance of said electroluminescent layer.

2. The electroluminescent display device set forth in claim 1 in which said resistance coating exhibits the property of a decrease of resistance in response to a decrease in voltage.

3. The electroluminescent display device of claim 2 in which said resistance coating is connected between adjacent back electrode members and the voltage difference between said adjacent back electrode members modifies the resistance of said resistance coating.

4. The device set forth in claim 1 in which said resistance coating is provided over the region of said second side of said electroluminescent layer which are not covered by said back electrodes and being in electrical contact with said back electrodes.

5. The device set forth in claim 1 in which said resistance coating exhibits the property of a reduction in resistance in response to radiation.

6. The device set forth in claim 5 in which said resistance coating is a photoconductive material responsive to the emission from said electroluminescent layer. 

2. The electroluminescent display device set forth in claim 1 in which said resistance cOating exhibits the property of a decrease of resistance in response to a decrease in voltage.
 3. The electroluminescent display device of claim 2 in which said resistance coating is connected between adjacent back electrode members and the voltage difference between said adjacent back electrode members modifies the resistance of said resistance coating.
 4. The device set forth in claim 1 in which said resistance coating is provided over the region of said second side of said electroluminescent layer which are not covered by said back electrodes and being in electrical contact with said back electrodes.
 5. The device set forth in claim 1 in which said resistance coating exhibits the property of a reduction in resistance in response to radiation.
 6. The device set forth in claim 5 in which said resistance coating is a photoconductive material responsive to the emission from said electroluminescent layer. 